The shortcut you've designed there will launch Channels, but on the iPhone itself. What you want to do instead, is have your shortcut send an API command to the Channels app that is running on the AppleTV. It may sound daunting but it's really easy to handle, once you get the right ingredients into place.
First you will need to know the IP address of your AppleTV. You can get that via the AppleTV, Settings → Network. Ideally, you can configure your network's router so that this device's IP address never changes, so these API commands you're sending can always reliably find their target. I use DHCP Reservations with my router but manually assigning a static IP on the device would work too. This is kind of a digression, but here's some reading about that which might help on that front, at least conceptually speaking, not knowing anything about your LAN. DHCP IP reservation or Set a Static IP address for a device - Google Nest Help
Once you know the IP address of the target AppleTV, here is the command you would have your shortcut send, as "Text" to have it tune into NBC on the Channels app that is running on that AppleTV:
http://192.168.1.99:57000/api/play/channel/4.1
(192.168.1.99 is my IP address, and my NBC channel is 4.1)
(so replace those with your own)
That's really all there is to it. Here's a screenshot of mine if it helps:
I'm using the Channels Beta (via TestFlight) but that shouldn't matter in this context.
By the way, I included those "1 second wait" commands because my TV's are relatively ancient and slow, lol. And my Channels DVR guide and library are large, sometimes needs a beat before I can control it via the API command. You may not need those, if your TV wakes up faster, or your Channels DVR launches faster from a cold boot.
Test it out, play around and see what works best for you and your own hardware, setup and flow. It's easy to experiment and customize and hopefully you find it fun, too.